In fast neutron nuclear reactors, the vessel is closed by a very thick horizontal plate pierced by a certain number of apertures through which pass the components of the reactor which are generally vertically disposed within the vessel.
At their upper part these components have a flange, of larger diameter than the diameter of the component, which, when the component is installed, centers in a seating of circular section provided inside the plate of the reactor. The flange nevertheless remains projecting above the upper surface of the plate of the reactor and the component is held in place by means of a flange, termed an "anti-flyoff" flange, one part of which bears on the upper horizontal surface of the flange of the component and another part of which is fixed by screws to the plate of the reactor.
Some of the components introduced vertically into the reactor vessel have very large dimensions and rest on the plate of the reactor via a flange whose outer diameter is about 3 m for fast neutron nuclear reactors as currently made. Examples of these components, termed "large components", are the primary pump, the intermediate exchanger and the pre-heating pipe for the heat transfer fluid which, in the case of fast neutron nuclear reactors, is generally liquid sodium.
Other components, termed "average components", have an outer flange-diameter of between 1 and 2 m. Examples of these are the integrated purifying apparatus for the heat transfer fluid, the sampling pump, the purification installation pump and the plugs giving access to the fuel.
Lastly, other components, termed "small components", have a diameter of less than one meter and are mounted in very great numbers on the plate of the reactor. A great many of these components constitute the instrumentation which it is necessary to dispose inside the vessel to carry out the various measurements or samplings in the reactor.
All of these components obviously have to be fixed sealingly onto the plate of the reactor so as to avoid liquid sodium vapor escaping from the vessel, on the one hand, and so as to avoid all radio-active contamination in the neighborhood of the vessel on the other hand. Sealing between the flanges of the components and the plate of the reactor has been achieved hitherto by the interposition of O rings between the lower surface of the flange and the upper surface of the seating in the plate in which this flange is positioned.
These sealing devices constituted by two O rings disposed concentrically around the component between the flange of the component and the plate of the reactor present disadvantages for which there has been no remedy hitherto.
For example, the flanges of large and average components whose diameter can be around two or three meters often present faults in flatness entailing faulty sealing of the device with O rings. These faults in flatness are inherent in the method of manufacturing these components and can be due for example to stresses produced in the component by welding.
Also, the large components are constructed in the horizontal position and when they are erected in order to be put into the vertical position in the reactor vessel, these components suffer a certain deformation which makes it very difficult to obtain a plane which is truly perpendicular to the axis of the component since, at the moment when this flange plane is established, the component is in the horizontal position.
In the case of nuclear reactors as currently made, this inclination of the flange plane to the axis of the component must be allowed for when it is sealed on being installed on the plate of the reactor.
Furthermore, with most nuclear reactor components, handling other than that necessary for positioning the component at the time of the reactor's construction occurs. During this handling, difficulties arise when the components are put back in position on the plate of the reactor, with interposition of the sealing O rings.
For example, the presence of condensates has been established, resulting from sodium vapor rising up the vents of components while the reactor is working, reaching the first O ring ensuring sealing and condensing on this seal.
Also, when the component is removed, particles of sodium oxide disposed in the annular seating provided in the plate sometimes fall on the seal seatings for locating the sealing O rings.
The condensates or the deposits on the seals are obviously deterimental to the correct repositioning of the components and are the cause of sealing faults.
In addition, removal, operations on the reactor components and remounting of these components gradually bring about a degradation in the state of the surface originally provided for supporting the seals and can be the reason why sealing faults gradually appear.
When a sealing fault is established, it is necessary to carry out extraction of the component in order to be able to change the seals and clean the seal seatings with special equipment comprising a protection plug and a special cleaning lock chamber so as to avoid contamination from contact with the environment of the reactor.
When the component is put back in position, particles from the handling head of the component can become disposed on the seal seatings and the new seals can become damaged in their turn.
Furthermore, sealing devices are known for elements which remain fixed on the plate of the reactor, constituted by a seal-bearing ring comprising two sets of seals, one of which effects sealing between the side faces of the ring and the element and the other of which effects sealing between the plate of the reactor and the lower surface of the seal-bearing ring. Such devices, however are not removable for easy fitting and removal of the element.